The mass flow rate of a fluid (defined by its average velocity multiplied by its mass density multiplied by the cross-sectional area of the channel through which the flow travels) is a measured quantity of interest in the control or monitoring of most practical and industrial applications, such as any chemical reaction, combustion, heating, cooling, drying, mixing, fluid power, etc. Generally speaking, a thermal anemometer is used to measure the mass velocity at a point or small area in a flowing fluid—be it liquid or gas. The mass velocity of a flowing fluid is its velocity referenced to standard or normal temperature and pressure. The mass velocity averaged over the flow channel's cross-sectional area multiplied by the cross-sectional area is the standard or normal volumetric flow rate through the channel and is a common way of expressing the total mass flow rate through the channel.
The thermal anemometer is sometimes referred to as an immersible thermal mass flow meter because it is immersed in a flow stream or channel in contrast to other thermal mass flow meter systems, such as those which sense the total mass flow rate by means of a heated capillary tube mounted externally to the flow channel. The operational principles of thermal anemometers derives from the fact that a heated sensor placed in a fluid stream transfers heat to the fluid in proportion to the mass flow rate of the fluid. In a thermal anemometer, one such heated sensor is provided together with another sensor that detects fluid temperature. In the constant-temperature mode of operation, the heated sensor is maintained at a constant temperature above the fluid temperature. The temperature difference between the flowing fluid and the heated sensor results in an electrical power demand in maintaining this constant temperature difference that increases proportional to the fluid mass flow rate and that can be calculated. Alternately, some thermal anemometers operate in a constant-current mode wherein a constant current or power is applied to the heated sensor and the fluid mass flow rate is calculated from the difference in the temperature of the heated sensor and the fluid temperature sensor, which decreases as mass flow rate increases. Thermal anemometers have greater application to gases, rather than liquids, because their sensitivity in gases is higher than in liquids.
Because the parts of the heated sensor of known thermal anemometers are not sufficiently reproducible dimensionally or electrically, known thermal anemometers require multi-point flow calibration of electrical output versus mass flow rate, usually in the actual fluid and with the actual ranges of fluid temperature and pressure of the application. For industrial applications, the heated sensor and fluid temperature sensor of known thermal anemometers typically have their respective sensors encased in a protective housing (e.g., thermowell or metallic tube sealed at its end, etc.). Usually, the encased heated sensor is inserted into the tip of the housing and is surrounded by a potting compound, such as epoxy, ceramic cement, thermal grease, or alumina powder.
In such a system, “skin resistance” and stem conduction are two major contributors to non-ideal behavior and measurement errors in thermal anemometers constructed in this manner. Skin resistance is the thermal resistance between the encased heated sensor and the external surface of the housing exposed to the fluid flow. The well-known hot-wire thermal anemometers have zero skirt resistance, but thermal anemometers with a housing do have skin resistance. The use of a potting compound substantially increases the skin resistance because such potting compounds have a relatively low thermal conductivity.
Skin resistance results in a temperature drop between the encased heated sensor and the external surface of the housing which increases as the electrical power supplied to the heated sensor increases. Skin resistance creates a “droop” and decreased sensitivity in the, power versus fluid mass flow rate calibration curve which is difficult to quantify and usually varies from meter to meter because of variations both in the parts of construction and in installation. The ultimate result of these skin-resistance problems is reduced accuracy. Furthermore, the use of a surrounding potting compound can create long-term measurement errors caused by aging and by cracking due to differential thermal expansion between the parts of the heated sensor.
Stem conduction causes a fraction of the electrical power supplied to the encased heated sensor to be passed through the stem of the heated sensor, down the housing, lead wires, and other internal parts of the heated sensor, and ultimately to the exterior of the fluid flow channel. Stem conduction couples the electrical power supplied to the encased heated sensor to the ambient temperature outside the channel. If the ambient temperature changes, stem conduction changes, and measurement errors occur. Similarly, stem conduction is responsible for errors in the encased fluid temperature sensor's measurement because it too is coupled to the ambient temperature.
Further discussion of the operational principles of known immersible thermal mass flow meters, their several configurations, particular advantages, uses, skin resistance, and stem conduction are presented in section 29.2 entitled “Thermal Anemometry” by the inventor hereof as presented in The Measurement Instrumentation and Sensors Handbook, as well as U.S. Pat. Nos. 5,880,365; 5,879,082; and 5,780,736, all assigned to Sierra Instruments, Inc., and each incorporated by reference herein in its entirety.
As noted in the referenced material, resistance temperature detectors (RTDs) may be employed in the heated sensor and the fluid temperature sensor, when one is provided. Alternative sensors for either the heated sensor or the fluid temperature sensor include thermocouples, thermopiles, thermistors, and semiconductor junction thermometers. RTD sensors are generally recognized as being more accurate and stable than any of these alternatives.
RTD sensors operate on the principle of electrical resistance increasing in accordance with increasing temperature. In known thermal anemometers, the RTDs are provided most commonly in the form of wire-wound sensors, but also as thin-film sensors (such as provided on an alumina chip) and least commonly as micro-machined sensors (such as provided in a silicon wafer). The most common wire-wound RTD sensors are usually manufactured via hand winding and hand resistance trimming, as well as other manual operations. This makes them vulnerable to human error in production and subject to irreproducibilities. The labor content, as well as the high cost of platinum wire, make them quite costly. Variations in the dimensions of the circular mandrel (e.g., alumina) over which the wire is typically wound and the insulating coating (e.g., glass) over the wound wires cause further dimensional and electrical-resistance irreproducibilities in wire-wound RTD sensors. Micro-machined RTD sensors have even worse dimensional and electrical resistance tolerances. As such, neither type of sensor is ideal for use in thermal anemometers.
On the other hand, thin-film RTD sensors are mass produced using automated production operations, employing technologies such as photolithography and lasers. This results in the comparatively high reproducibility, accuracy, stability, and cost-effectiveness of thin-film RTD sensors. Yet, prior to the teaching offered by the present invention, some thermal anemonmeters have used thin-film RTD's that were not entirely encased in a protective housing and which had their surfaces directly exposed to the fluid. Due to the fragility, poorer dimensional tolerances, and the oscillating and turbulent flow around the thin-film RTD body, etc., such devices—standing alone—have only proven suitable for light duty, low-end, low-accuracy/precision requirement applications.
Prior to the solution offered by the present invention, the best accuracy typically achievable in current thermal anemometers for industrial applications was approximately 2% to 3% of reading error in accuracy over a mass flow rate range of 10% to 100% of full scale and over a relatively smaller temperature and pressure range. The construction of the heated sensor selected is what limits the accuracy. Most commonly, a wire-wound RTD sensor and, less commonly, a thin-film RTD sensor is encased in the tip of a metallic tube (e.g., 316 stainless steel) sealed at its end and surrounded by a potting compound (e.g., epoxy, ceramic cement, thermal grease, or alumina powder).
Sensor fabrication with such potting compounds is inherently irreproducible due to variations in their composition, amount used, insufficient wetting of surfaces, and/or air bubbles. In the case of wire-wound sensors, this irreproducibility is added to previously mentioned irreproducibilities associated with wire-wound RTD sensors themselves. These irreproducibilities, combined with the previously mentioned high skin resistance and potential for long-term instability associated with the use of potting compounds, limits the-overall accuracy of known thermal anemometers constructed in this manner.
The thermal anemometer described in U.S. Pat. No. 5,880,365 avoids the accuracy degrading use of potting compounds by forming the encasing housing over the wire-wound RTD sensor by means of forces external to the housing. This construction has high stability and improved accuracy but is relatively expensive and may have irreproducibilities associated both with wire-wound RTD sensors and with variations from meter to meter in the gap between the wire-wound RTD and the internal surface of the housing.
However, the present invention employs a thin-film RTD not prone to such problems. It does so in a manner not heretofore contemplated, thereby offering the advantage of the sensor type's relative benefits, but in a highly accurate meter. As such, the present invention offers a significant advance in the art.